Antennas are used in sensors, radars and radio communication systems to transmit and/or receive electromagnetic signals wirelessly at frequencies over which the antenna element(s) experience electromagnetic resonance. Resonant dipole antennas are a class of antennas where the electromagnetic radiation emissivity/sensitivity is pronounced at the antenna's fundamental frequency and harmonics of the fundamental frequency. Resonant dipoles have low to moderate gain, which is useful in transceiver systems that require general insensitivity to the relative direction (and/or orientation) of transmit and receive antennas, such as mobile communications. They also have relatively high efficiency at resonance, which is commonly represented as a low return loss. In general, a dipole antenna spanning a length (Z) will exhibit its fundamental resonance frequency ƒfund (also known as the first harmonic) over electromagnetic emissions having wavelength(s) given by:2l≅λfund  (1)
TABLE 1Required Communications Frequency BandsCountryUMTSGSMEurope2100 900United States/Canada 850 or 1700 or 21001900 or 850 China2100 900Japan2100(not supported)Argentina 8501900Brazil21001800Chile 850 or 1900 850 or 1900India2100 900Egypt2100 900South Africa2100 900
As shown in FIG. 1, a 0.5 m long dipole antenna will have its fundamental frequency 1 close to 300 MHz and harmonic resonances 2A,2B at odd integer multiples (900 MHz and 1500 MHz) of the fundamental frequency 1. Although dipole antennas have some desirable characteristics for mobile device applications, such as low to moderate gain and high efficiency (low return loss), their conductive pass bands 1, 2A, 2B do not align with the allocated communication frequency bands (UMTS 1700, UMTS 1900, UMTS 2100, GPS, GSM 850, GSM 900, GSM 1700, GSM 1800, and WiFi) typically used by these devices. As a consequence, multiple antenna elements are required to cover the frequency spectrum requirements of a typical mobile communications device. Table 1 shows the required frequency bands for cellular communications using voice, text, and mobile data over Universal Mobile Telecommunications Systems (UMTS) third-generation (3G) systems in various countries around the world, as well as the required frequency bands for cellular communications using voice, text and mobile data over Global System for Mobile Communications (GSM) second-generation (2G) systems in those countries. Most countries recommend supporting a larger number of frequency bands than those shown in TABLE 1 depending upon the size of its geographic territory and/or telecommunications market. The larger number of frequency bands allows multiple carriers (service providers) to supply the national population and bid for premium (required) bands in regions where they have higher customer concentrations, while lowering carrier costs by using lower value (recommended) bands in regions where their customer concentration is less strong.
As a result of this general landscape within the industry, a single service provider will likely require mobile wireless devices that contain multiple antennas/radio systems to faultlessly navigate its domestic territory or provide global portability. The better broadband antennas will electrically communicate with 33% bandwidth (Δf/fcenter) and have a peak efficiency of 70-80%, where Δf=fupper−flower. These broadband antennas would allow a single antenna element to cover two bands that are closely positioned in frequency, such as the GSM 1700 and GSM 1800 bands (see TABLE 2), but not all the frequency bands at which the mobile wireless unit must communicate and certainly not at peak efficiency. Multiple antenna elements are undesirable since each element adds to the overall cost and occupied volume.
TABLE 2Select Frequencies of Cellular Communications BandsFrequency BandUplink (MHz)Downlink (MHz)UMTS21001920-19802110-217019001850-19101930-19901700 IX1749.9-1784.91844.9-1879.91700 X1710-17702110-2170GSM19001850.2-1910.21930.2-1990.218001710.2-1785.81805.2-1879.8 900880-915925-960 850824-849869-894
Filtering components are electrically coupled with the antenna system in the RF front-end to isolate specific frequency bands of interest for a given transceiver (radio/radar) application. The filtering components prevent electromagnetic emissions that fall outside of the desired frequency range(s) from interfering with the signal(s) of interest and are generally required to isolate the chosen frequency band from any undesirable frequency emissions to a level −40 dB or more in most applications. As shown in TABLE 2, mobile communications system designate a portion (subband) of the communications band for uplink frequencies (from the mobile device to the tower) and another portion for downlink frequencies (from the tower to the mobile device). The RF front-end must fully isolate these distinct signaling frequencies from one another and operate simultaneously if full duplex mode communications is desired. Acoustic-wave filters are generally used in cellular communications systems to isolate uplink frequencies 3 from downlink frequencies 4 and provide the requisite better than −40 dB signal isolation as shown in FIGS. 2A&2B. In addition to adding cost to and occupying space on the mobile platform, acoustic-wave filters will contribute 1.5 dB to 3 dB insertion loss between the antenna and the send/receive circuitry. Higher insertion losses are undesirable as they divert the available power to the radio and away from other useful functions.
Mobile wireless devices have radios with fixed frequency tuning, so a single radio system will only communicate over a specific frequency band. As a result of the fixed uplink/downlink tuning most mobile devices will have multiple radio systems since a given wireless carrier may not have license to operate at the premium (required) frequency bands shown in TABLE 1 throughout an entire nation. A given wireless service provider will be less likely to have access to the premium or required frequencies in foreign countries. The need for additional radios in their mobile systems is undesirable as it adds considerable cost to the service.